Yep - when the feathery bit dies back a bit as thisisyesterday said. You can check by opening one up a little, push your nail into a corn, if it oozes white milky stuff they are ready. Pick them as you want them - don't store them as they spoil. Best thing is to have a pan boiling, run out and pick then drop straight in the pan - YUM!!!!

No you snap off the one that is ready, there might be 2 or 3 one one plant, depending on variety they may not all be ready together. To snap off, grab it and pull down then away, should come away cleanly if it is ready ime.

oh shitty bumsso, if i have picked 3 cobs and they are in fact, not ripe , then is there a way to ripen them? leave em in the greenhouse and hope for the best? [knows answer is no, and thinks about cooking anyway and giving to the chickens]

Bugger I did think they were early but I did not know if there was an early variety. Give them a few more weeks, the tassles should have died back (not just droopy) and the outer covering starting to go a bit "crispy".